Well, it's not a DPMS barrel, as ours are marked entirely differently. It's possible that it was one that was purchased from us many years ago, but that marking is a fairly generic one, and there's no way to be certain of it's origin as being here.
As to your problem, if you've tried different mags and ammo, then you've pretty much eliminated that. If you haven't done so already, give the rifle a simple once over and check it for (like you said) loose parts, gas rings in place, everything lining up properly, etc.. Beyond that, you might consider having a reputable gunsmith take a look and check gas port alignement, size, and barrel extension alignment.
You also might just consider contacting the dealer and asking how he'll handle it for you, since it's not a factory built upper (at least OUR factory).